Heat exchanger with an improved configuration of passages, associated methods for exchanging heat

ABSTRACT

A heat exchanger having a plurality of plates parallel to a longitudinal direction and together defining a first series of passages for the flow of at least one refrigerant configured to exchange heat with at least one calorigenic fluid, at least one passage of the first series defined between two adjacent plates

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a 371 of International Application No. PCT/FR2019/051779, filed Jul. 16, 2019, which claims priority to French Patent Application No. 1857133, filed Jul. 31, 2018, the entire contents of which are incorporated herein by reference.

BACKGROUND

The present invention relates to a heat exchanger comprising series of passages to allow the flow of at least one refrigerant to exchange heat with a calorigenic fluid, the exchanger comprising at least one passage configured to allow the flow of said refrigerant and of at least one other refrigerant.

The technology currently used for an exchanger is that of aluminum brazed plate-fin exchangers, which allow highly compact devices to be obtained offering a large exchange surface.

These exchangers comprise a stack of plates that extend in two dimensions, namely length and width, thus forming a stack of vaporization passages and of condensation passages, respectively intended, for example, to vaporize the refrigerant and to condense a calorigenic gas. It is to be noted that the exchanges of heat between the fluids can occur with or without a change of phase.

In order to introduce and discharge the fluids into and out of the exchanger, the passages are provided with fluid inlet and outlet openings. The inlets and outlets placed one above the other in the stacking direction of the passages of the exchanger are respectively joined at inlet and outlet manifolds of general semi-tubular shape, through which the fluids are distributed and discharged.

Several calorigenic and refrigerant fluids, of a distinct nature and/or with distinct features, can circulate in the exchanger. These fluids form distinct streams or flows that are introduced into and discharged from the exchanger via sets of inlets and outlets dedicated to one type of fluid.

Conventionally, in the event that several refrigerants circulate in the exchanger, the inlets and outlets for the different refrigerants are successively arranged, along the length of the exchanger, in increasing order of temperature, starting from the cold end of the exchanger, i.e. the point of entry into the exchanger where a fluid is introduced at the lowest temperature of all the temperatures of the exchanger.

Thus, when the outlet temperature of a refrigerant is higher than the inlet temperature of another refrigerant, the other refrigerant must enter the exchanger, along the length of the exchanger, at a position that is closer to the cold end than the outlet of the refrigerant.

In a known manner, the pinch analysis method is used to plan how the fluids exchanging heat in the exchanger circulate and how to maximize the energy efficiency of the installation.

The term “pinch” refers to the minimum difference between the temperature of the refrigerants, i.e. the fluids that heat in the exchanger, and the temperature of the calorigenic fluids, i.e. the fluids that cool in the exchanger, with this being at a given point of the exchanger. In order to see this pinch point, the difference between two composite curves of an exchanged heat-temperature diagram is analyzed, as illustrated in FIG. 3B (a), one being associated with the flow to be heated, the other with the flow to be cooled. As long as this minimum difference is positive, then theoretically there is a way to reduce energy consumption.

Conventionally, in order to optimize the pinch point between the curves of the exchange diagram originating from the pinch analysis method, at least two types of different refrigerant passages are provided, one type of passage dedicated to the circulation of a refrigerant and at least one other type of passage dedicated to the circulation of the other refrigerant. These different types of passages are not formed between the same pair of adjacent plates of the exchanger.

This increases the complexity of the exchanger and significantly increases the size of the exchanger. Furthermore, each type of passage then has a significant portion in which no fluid circulates, i.e. an inactive zone in terms of the exchange with the calorigenic fluid.

SUMMARY

The aim of the present invention is to overcome all or some of the aforementioned problems, in particular by proposing a heat exchanger that is more compact and has improved thermal efficiency and mechanical strength.

Therefore, the solution according to the invention is a heat exchanger comprising a plurality of plates parallel to a longitudinal direction and together defining a first series of passages for the flow of at least one refrigerant intended to exchange heat with at least one calorigenic fluid, at least one passage of the first series defined between two adjacent plates comprising:

-   -   a refrigerant inlet configured to introduce the refrigerant into         a portion of said passage and a refrigerant outlet configured to         discharge the refrigerant from the portion,

characterized in that said at least one passage of the first series further comprises:

-   -   at least one other refrigerant inlet configured to introduce         another refrigerant into another portion of said passage and at         least one other refrigerant outlet configured to discharge the         other refrigerant from the other portion,

said other inlets and outlets being arranged so that said at least one passage of the first series is divided, in the longitudinal direction, into at least said portion for the flow of the refrigerant and said other portion for the flow of the other refrigerant.

Depending on the situation, the invention can comprise one or more of the following features:

-   -   a plurality of passages of the first series each comprises at         least one inlet, one outlet, one other inlet and one other         outlet for refrigerant, said inlets being fluidly connected to         the same inlet manifold, said other inlets being fluidly         connected to another identical inlet manifold, said outlets         being fluidly connected to an identical outlet manifold and said         other outlets being fluidly connected to another identical         outlet manifold;     -   the exchanger comprises a first end, in the vicinity of which,         during operation, the temperature is the lowest of the         exchanger, and a second end, in the vicinity of which, during         operation, the temperature is the highest of the exchanger, said         second end being arranged downstream of the first end in the         longitudinal direction, the portion for the flow of the         refrigerant being arranged alongside the first end and the other         portion for the flow of the other refrigerant being arranged         between the portion and the second end;     -   the plates together define a second series of passages for the         flow of at least one calorigenic fluid, at least one passage of         the second series being adjacent to said at least one passage of         the first series and being configured so that, when a stream of         calorigenic fluid circulates in said passage, said stream of         calorigenic fluid exchanges heat with the refrigerant in the         vicinity of at least part of the portion and with the other         refrigerant in the vicinity of at least part of the other         portion;     -   at least one passage of the second series comprises, in the         vicinity of the second end of the exchanger, a calorigenic fluid         inlet configured to distribute the calorigenic fluid in said at         least one passage and, in the vicinity of the first end of the         exchanger, an outlet configured to discharge the calorigenic         fluid from said at least one passage;     -   said at least one passage of the first series comprises at least         two other inlets configured to respectively introduce at least         two other refrigerants into at least two other respective         portions of said passage and at least two other outlets         configured to respectively discharge the at least two other         refrigerants from the at least two other portions, said at least         two other inlets and at least two other outlets being arranged         so that said at least one passage of the first series is         divided, in the longitudinal direction, into at least three         successive portions.

The invention also relates to a method for exchanging heat implementing a heat exchanger according to the invention, said method comprising the following steps:

-   -   i) introducing a stream of calorigenic fluid into at least one         passage of a second series of passages defined between the         plates of the exchanger;     -   ii) introducing a refrigerant via the inlet of at least one         passage of the first series;     -   iii) discharging the refrigerant introduced in step ii) via the         outlet of said passage;     -   iv) introducing at least one other refrigerant via said other         inlet of said passage;     -   v) discharging the other refrigerant introduced in step iv) via         the other outlet of said passage,

said stream of calorigenic fluid exchanging heat at least with the refrigerant and with the other refrigerant.

Preferably, the refrigerant discharged in step iii) has a first temperature and the other refrigerant introduced in step iv) has a second temperature, with the second temperature being lower than the first temperature.

Advantageously, the second temperature is at least 1° C. lower than the first temperature.

The present invention can be applied to a heat exchanger that vaporizes at least two partial streams of a fluid with two liquid-gas phases as refrigerants, in particular at least two partial streams of a mixture with a plurality of constituents, for example, a mixture of hydrocarbons, by exchanging heat with at least one calorigenic fluid, for example, natural gas.

In particular, the invention can be applied to a method for cooling, even liquefying, a mixture of hydrocarbons such as natural gas. In particular, the liquefying method is implemented in a method for producing liquefied natural gas (LNG).

The expression “natural gas” relates to any composition containing hydrocarbons, at least including methane. This comprises a “crude” composition (prior to any treatment or scrubbing) and also any composition which has been partially, substantially or completely treated for the reduction and/or removal of one or more compounds, including, but without being limited thereto, sulfur, carbon dioxide, water, mercury and certain heavy and aromatic hydrocarbons.

Thus, the invention relates to a method for cooling a stream of hydrocarbons, such as natural gas, as a stream of calorigenic fluid, said method implementing a heat exchanger according to the invention or a method for exchanging heat according to the invention and comprising the following steps:

-   -   a) introducing the stream of hydrocarbons into the heat         exchanger;     -   b) introducing a first cooling stream into the heat exchanger;     -   c) extracting, from the heat exchanger, a partial cooling stream         and at least one other partial cooling stream originating from         the first cooling stream;     -   d) relieving the partial cooling stream and the other partial         cooling stream at different pressure levels in order to         respectively produce the refrigerant and the other refrigerant;     -   e) reintroducing the refrigerant produced in step d) via the         inlet of at least one passage of the first series;     -   f) reintroducing the other refrigerant produced in step d) via         the other inlet of said passage;     -   g) cooling the stream of hydrocarbons by exchanging heat with at         least the refrigerant and the other refrigerant.

It is to be noted that the stream of hydrocarbons originating from step g) can be at least partly liquefied.

Optionally, the stream of hydrocarbons cooled and/or at least partially liquefied in step g) is introduced into another exchanger, into which a second cooling stream is introduced. Preferably, the second cooling stream exiting the other exchanger is relieved, then reintroduced into said other exchanger in order to be vaporized therein whilst cooling the stream of hydrocarbons and the second cooling stream, so that the stream of hydrocarbons exits the other exchanger liquefied and subcooled.

The first cooling stream can be a mixture of hydrocarbons, for example, a mixture containing ethane and propane.

Preferably, the refrigerant produced in step d) has a first pressure and the other refrigerant produced in step d) has a second pressure, the second pressure being higher than the first pressure.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be better understood by virtue of the following description, which is provided solely by way of a non-limiting example and with reference to the accompanying drawings, in which:

FIG. 1 is a schematic section view, in a plane parallel to the plates of the exchanger, of a refrigerant passage of a heat exchanger according to the prior art;

FIG. 2 is a schematic section view, in a plane orthogonal to the plates and parallel to the longitudinal direction of the exchanger, of series of passages of the heat exchanger of FIG. 1;

FIG. 3A is another schematic section view, in a plane parallel to the plates of the exchanger, of a passage of a heat exchanger according to another embodiment of the invention;

FIG. 3B illustrates, on the one hand, the exchange diagram curves of a conventional exchanger as illustrated in FIG. 1 and, on the other hand, the exchange diagram curves of an exchanger according to the invention as illustrated in FIG. 3A;

FIG. 4 schematically shows an embodiment of a method for exchanging heat implementing an exchanger according to one embodiment of the invention;

FIG. 5 is a schematic section view, in a plane parallel to the plates of the exchanger, of a passage of a heat exchanger according to another embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates the passages 10 a, 10 b of a heat exchanger according to the prior art comprising a plurality of plates 2 that extend in two dimensions, namely length and width, respectively in a longitudinal direction z and a lateral direction y orthogonal to z and parallel to the plates 2.

The plates are disposed in parallel one above the other with spacing in a stacking direction x, thus forming a plurality of passages for fluids indirectly exchanging heat via the plates. Preferably, each passage of the exchanger has a parallelepiped and flat shape. The gap between two successive plates is small compared to the length and the width of each successive plate.

FIG. 1 schematically shows the passages of an exchanger configured to vaporize a refrigerant F1 and another refrigerant F2 by exchanging heat with a calorigenic fluid C.

It is to be noted that the other refrigerant F2 can be a fluid with a different composition than the refrigerant F1 or even a refrigerant with the same composition as the refrigerant F1 but at least one physical feature, in particular pressure, temperature, that is different from that of the refrigerant F1.

The calorigenic fluid C circulates in a second series of passages 11 (shown in FIG. 2) fully or partly arranged alternately or adjacently with all or some of the passages 10 a, 10 b of the first series, The flow of the fluids in the passages occurs generally parallel to the longitudinal direction z, which is preferably, as in the illustrated case, vertical during the operation of the exchanger.

The seal of the passages 10 a, 10 b along the edges of the plates is generally provided by lateral and longitudinal sealing strips 4 attached onto the plates. The lateral sealing strips 4 do not completely seal the passages 10 a, 10 b but leave fluid inlet 31, 32 and outlet 41, 42 openings.

In a manner per se known, the exchanger comprises distribution components 51, 61, 52, 62 that extend from and to the inlets and outlets of the passages. These components, for example, distribution waves or channels, are configured to direct and to provide even distribution and recovery of the fluids over the entire width of the passages.

Furthermore, the passages 10 a, 10 b advantageously comprise heat exchange structures disposed between the plates. The purpose of these structures is to increase the heat exchange surface area of the exchanger. Indeed, the heat exchange structures are in contact with the fluids circulating in the passages and transfer thermal flows by conduction up to the adjacent plates.

The heat exchange structures also act as spacers between the plates 2, in particular during the brazed assembly of the exchanger and to avoid any deformation of the plates when implementing pressurized fluids. They also provide guidance for the flows of fluid in the passages of the exchanger.

Preferably, these structures comprise heat exchange waves that advantageously extend along the width and the length of the passages 10 a, 10 b, parallel to the plates 2, in the extension of the distribution components 51, 61, 52, 62 along the length of the passages 10 a, 10 b. The passages of the exchanger thus have a main part of their length that forms the heat exchange zone per se, which is bounded by distribution zones provided with the components 51, 61, 52, 62.

Such an arrangement of passages according to FIG. 1 is particularly encountered in an exchanger implemented in a method for liquefying natural gas. One of the known methods for obtaining liquefied natural gas is based on the use of two cycles for cooling the natural gas respectively implementing a first and a second mixture of cooling hydrocarbons. The first cooling cycle allows the natural gas to be cooled to its dew point using at least two different relief levels for increasing the efficiency of the cycle. The second cycle allows the natural gas to be liquefied and subcooled and only has one relief level.

In the first relief cycle, the first cooling mixture originating from a compressor is subcooled in a first exchanger. At least two partial streams originating from the first cooling mixture are extracted from the exchanger at two distinct exit points, then relieved at different pressure levels, thus forming at least two distinct refrigerants F1 and F2 reintroduced into the exchangers via distinct inlets 31, 32 selectively feeding the passages 10 a, 10 b in order to be vaporized therein, then discharged via distinct outlets 41, 42.

According to the known method, the refrigerant F1 relieved at a given pressure level enters via the inlet 31 located at the cold end of the exchanger and exits via the outlet 41 at a temperature higher than the inlet temperature via the inlet 32 of the other refrigerant relieved at another pressure level.

In order to follow the arrangement of the inlets and outlets in an increasing order of temperature of the fluids, the inlet of the other refrigerant is conventionally located, in the longitudinal direction z, at a position closer to the cold end of the exchanger than the outlet of the lower pressure refrigerant.

As can be seen in FIG. 1, the exchanger comprises two types of refrigerant passages, one 10 a for the refrigerant F1 and the other 10 b for the other refrigerant F2, The calorigenic fluid C flowing in the passages 11 adjacent to the passages of one type 10 a and/or of another type 10 b therefore exchange heat in the vicinity of the active exchange zone A1 with the fluid F1 and in the vicinity of the active exchange zone A2 for the other fluid F2. The zones I1 and I2 are not fed with fluid and therefore form inactive zones on the thermal level.

The present invention aims to reduce the longitudinal extent of these inactive zones, and even to completely eliminate them by proposing the longitudinal sharing of at least one passage formed between two plates 2 of the exchanger and to cause different refrigerants to circulate therein.

FIG. 3 is a schematic section view, in a plane parallel to that of FIG. 1, of a passage of an exchanger according to one embodiment of the invention. In the illustrated example, the number of different types of refrigerants is limited to 2 for the sake of simplification, with it being understood that a greater number of types of fluid could circulate in such a passage in accordance with the same principle.

As can be seen in FIG. 3, at least one passage 10 of the first series of refrigerant passages comprises at least one other inlet 32 and at least one other outlet 42 for another refrigerant F2.

Said other inlets and outlets 32, 42 are arranged so that said passage 10 of the first series is divided, in the longitudinal direction z, into at least one portion 100 for the flow of the refrigerant F1 and another portion 200 for the flow of the other refrigerant F2.

In this way, when the exchanger operates, several different types of refrigerants F1, F2 circulate inside the same passage, i.e. between the same two plates of the exchanger, on dedicated flow portions that follow one another in the longitudinal direction z.

In this way, the proportion of inactive zones in the exchanger is significantly reduced, even eliminated, with the same passage having at least two active exchange zones A1, A2, in the vicinity of which the refrigerant F1 and said at least one other refrigerant F2 successively exchange heat with the calorigenic fluid C.

Nearly all, even all, of a calorigenic fluid passage 11 of the second series is thus in contact with a refrigerant passage 10 of the first series, which promotes the heat exchange and drastically reduces the thermal and mechanical stresses exerted on the plates and the inlet/outlet manifolds of the exchanger. The size of the exchanger can be reduced, thus reducing the cost of the exchanger and of the cold box in which it is integrated. The reduction of the inactive zones inside the exchanger also increases its mechanical strength.

In fact, the inventors of the present invention have demonstrated that by taking into account the temperature overlaps from the design phase of the method, it is possible to circulate the refrigerants in the same passage, even if the outlet temperature of the first fluid is higher than the inlet temperature of the second fluid. To this end, the exchanger needs to be simulated, not as a single section with two refrigerants arriving at different temperatures, as is the case with the known pinch analysis method, but as different consecutive sections (two in the cited example). with each of these sections comprising a single refrigerant, arriving at its inlet temperature, in order to best approach the actual geometry and therefore the actual pinch points that the exchanger will exhibit.

This is illustrated in FIG. 3B, which shows a comparison between the exchanged heat-temperature (ΔH-T) exchange diagrams, or enthalpic curves, obtained, on the one hand, with an exchanger simulated according to the conventional pinch analysis method (in (a)) and, on the other hand, with an exchanger in which the fluids circulate in accordance with the invention (in (b)). The curves C, F, F1, F2 illustrate the evolution of the amount of heat that is exchanged as a function of the temperature, respectively for the calorigenic fluid, a composite refrigerant constructed using the conventional pinch analysis method, the refrigerant F1 according to the invention and the other refrigerant F2 according to the invention.

Preferably, the majority, more preferably at least 80% of the total number of passages 10 of the first series, even all the passages 10 of the first series, each comprise an inlet and an outlet 31, 41 for the refrigerant F1 and at least one other inlet and one other outlet 32, 42 for the other refrigerant F2.

Advantageously, the exchanger according to the invention has a single type of refrigerant passage 10, which significantly simplifies the design. Passages of the same type are understood to be passages that have an identical configuration or structure, in particular in terms of dimensions of the passages, arrangements of he fluid inlets and outlets.

Preferably, the majority, preferably at least 80%, even all, of the total number of passages 10 of the first series have an identical configuration. In particular, the inlets and outlets 31, 41, 32, 42 are arranged at substantially identical positions in the longitudinal direction z.

Thus, the inlets and outlets 31, 41, 32, 42 of the passages 10 of the first series are correspondingly disposed one above the other, following the stacking direction x of the passages. The inlets 31, 32 and outlets 41, 42 thus placed one above the other are respectively joined in semi-tubular shaped manifolds 71, 72, 81, 82, through which the fluids are distributed and discharged.

Preferably, the longitudinal direction is vertical when the exchanger is operating. The refrigerants F1, F2 generally flow vertically and upwardly. The calorigenic fluid C preferably circulates against the flow. Other directions and senses for the flow of the fluids F1, F2 obviously can be contemplated, without departing from the scope of the present invention.

Preferably, the passage 10 of the exchanger comprises distribution zones 51, 61, 52, 62, preferably provided with distribution components, which extend from and to the inlets 31, 32 and outlets 41, 42 of the passage 10. These distribution zones are configured to evenly direct and recover the fluids F1 and F2 over the entire width of the exchange zones A1 and A2, respectively.

Advantageously, the portion 100 of the passage 10 comprises the distribution zones 51, 61 and the exchange zone A1, and the other portion 200 comprises the distribution zones 52, 62 and the exchange zone A2.

Advantageously, heat exchange structures are arranged in the exchange zones A1 and A2. The various types of waves can be used that are commonly implemented in the brazed plate-fin type exchangers for forming the heat exchange structures of the exchange zones A1, A2. The waves can be selected from the types of wave known as straight waves, serrated waves, herringbone waves, which may or may not be perforated.

Advantageously, the distribution components and the heat exchange structures form a plurality of channels inside the passage 10 fluidly connecting the inlets 31 and outlets 41 together and the other inlets 32 and outlets 42 together.

Advantageously, the exchanger comprises a first end 1 a, in the vicinity of which, during operation, the temperature is the lowest of the exchanger, and a second end 1 b, in the vicinity of which, during operation, the temperature is the highest of the exchanger.

Preferably, the second end 1 b is arranged downstream of the first end 1 a in the longitudinal direction z, so that the direction of flow of the fluids F1, F2 in the passage 10 is generally ascending.

Preferably, the portion 100 for the flow of refrigerant F1 is arranged alongside the first end 1 a and the other portion 200 for the flow of the other refrigerant F2 is arranged between the portion 100 and the second end 1 b.

Thus, in the representation provided in FIG. 3, the other portion 200 extends, in the longitudinal direction z, downstream of the portion 100.

Preferably, the portions 100, 200 are juxtaposed in the longitudinal direction z, as illustrated in FIG. 3, which allows the space inside the passage 10 to be best optimized by maximizing the extent of the active zones.

According to an alternative embodiment, illustrated in FIG. 5, two other refrigerants F2, F3 flow in the same passage 10 according to the invention.

In this case, at least one refrigerant passage 10 of the first series comprises two other inlets 32, 33 configured to respectively introduce two other refrigerants F2, F3 into two other respective portions 200, 300 of the passage 10, and two other outlets 42, 43 configured to respectively discharge the other two refrigerants F2, F3 from the other two portions 200, 300. The passage 10 is divided, in the longitudinal direction z, into three successive portions 100, 200, 300.

Advantageously, when the exchanger operates, the refrigerant F1 enters via the inlet 31 of at least one passage 10 at a temperature, called initial temperature T0, and is discharged via the outlet 41 at a first temperature T1 higher than T0. Preferably, the temperature T0 ranges between −55 and −75° C. and the temperature T1 ranges between −10 and −30° C.

Preferably, the other refrigerant F2 enters the passage 10 via the other inlet 32 at a second temperature T2 and exits via the other outlet 42 at a third temperature T3, with T3 being higher than T2. Preferably, the temperature T2 ranges between −15 and −35° C. and the temperature T3 ranges between 35 and 0° C.

Preferably, the second temperature T2 is below the first temperature T1. This allows a fluid F1 to be provided that is overheated when exiting the portion 100 of the exchanger (T1 high), whilst providing effective cooling of the calorigenic fluid in the other portion 200 of the exchanger by virtue of a low enough (lower than T1) vaporization start temperature, T2, of the fluid F2.

More preferably, the second temperature T2 is at least 1° C. lower than the first temperature T1. Preferably, the second temperature T2 is at most 15° C., more preferably at most 10° C., and preferably at most 5° C. lower than the first temperature T1. This is in order to avoid excessive mechanical stresses in the exchanger.

Reference will now be made to the alternative embodiment where two other refrigerants F2, F3 flow in the same passage 10.

Advantageously, when the exchanger operates, the refrigerant F1 enters via the inlet 31 of at least one passage 10 at an initial temperature T0 ranging between −55 and −75° C. and is discharged via the outlet 41 at a first temperature T1 higher than T0, with T1 ranging between −25 and −45° C.

Preferably, the first other refrigerant F2 enters the passage 10 via a first other inlet 32 at a second temperature T2 and exits via the other outlet 42 at a temperature T3, with T3 being higher than T2. Preferably, the temperature T2 ranges between −30 and −50° C. and the temperature T3 ranges between 0 and −20° C.

Preferably, the second other refrigerant F3 enters the passage 10 via another second inlet 33 at a fourth temperature T4 and exits via another second outlet 43 at a fifth temperature T5, with T5 being higher than T4. Preferably, the temperature T4 ranges between −5 and −25° C. and the temperature T5 ranges between 30 and 0° C.

Advantageously, the fourth temperature T4 is lower than the third temperature T3. This allows a fluid F2 to be provided that is overheated when exiting the portion 200 of the exchanger (T3 high), whilst providing effective cooling of the calorigenic fluid in the other portion 300 of the exchanger by virtue of a low enough (lower than T3) vaporization start temperature, T4, of the fluid F3.

Preferably, the fourth temperature T4 is at least 1° C. lower than the third temperature T3.

Preferably, the second temperature T2 is at most 15° C., more preferably at most 10° C., and preferably at most 5° C. lower than the first temperature T1.

Advantageously, the fourth temperature T4 is at least 1° C. lower than the third temperature T3, preferably, the fourth temperature T4 is at most 15° C. lower than the third temperature T3, more preferably, in order to avoid excessive mechanical stresses in the exchanger, at most 10° C., and preferably at most 5° C. lower than the third temperature T4.

According to a particular embodiment, the refrigerant F1 and the at least one other refrigerant F2 are fluids with different pressures. In particular, the refrigerant F1 flows in the exchanger at a first pressure P1 and the other refrigerant F2 flows in the exchanger at a second pressure P2, which preferably is higher than the first pressure P1. The fluids F1, F2 can have the same composition.

An exchanger according to the invention can be used in any method implementing several different types of refrigerants, in particular in terms of composition and/or features such as pressure, temperature, physical state, etc.

The use of an exchanger according to the invention is particularly advantageous in a method for liquefying a stream of hydrocarbons, such as natural gas. An example of such a method is partially schematically shown in FIG. 4.

According to the method for liquefying natural gas schematically shown in FIG. 4, the natural gas arrives via the pipe 110, for example, at a pressure ranging between 4 MPa and 7 MPa and at a temperature ranging between 30° C. and 60° C. The natural gas circulating in the pipe 110, the first cooling stream circulating in the pipe 30 and the second cooling stream circulating in the pipe 20 enter the exchanger E1 according to the invention in order to circulate therein in parallel and co-current directions.

The natural gas exits the exchanger E1 via the pipe 102 cooled, for example, to a temperature ranging between −35° C. and −70° C. The second cooling stream exits the exchanger E1 via the pipe 202 completely condensed, for example, at a temperature ranging between 35° C. and 70° C.

In the exchanger E1, three fractions, also called flows or partial streams, 301, 302, 303 of the first liquid phase cooling stream are successively extracted. The fractions are relieved through the relief valves V11, V12 and V13 at three different pressure levels, forming a refrigerant F1 and two other refrigerants F2, F3. These three different types of refrigerants F1, F2, F3 are reintroduced into the exchanger E1 with refrigerant passages provided with three distinct inlets 31, 32, 33 according to the invention, then vaporized by exchanging heat with the natural gas, the second cooling stream and some of the first cooling stream.

The three vaporized refrigerants F1, F2, F3 are sent to different stages of the compressor K1, compressed, then condensed in the condenser C1 by exchanging heat with an external cooling fluid, for example, water or air. The first cooling stream originating from the condenser C1 is sent into the exchanger E1 via the pipe 30. The pressure of the first cooling stream at the outlet of the compressor K1 can range between 2 MPa and 6 MPa. The temperature of the first cooling stream at the outlet of the condenser C1 can range between 10° C. and 45° C.,

The first cooling stream can be formed by a mixture of hydrocarbons, such as a mixture of ethane and of propane, but can also contain methane, butane and/or pentane. The mole fraction proportions (%) of the components of the first cooling mixture can be:

-   -   Ethane: 30% to 70%     -   Propane: 30% to 70%;     -   Butane: 0% to 20%;

The natural gas circulating in the pipe 102 can be split, i.e. some of the hydrocarbons C₂₊ containing at least two carbon atoms is separated from the natural gas using a device that is known to a person skilled in the art. The split natural gas is sent via the pipe 102 into another exchanger E2. The collected hydrocarbons C2+ are sent into fractionating columns comprising a de-ethanizer. The light fraction collected at the top of the de-ethanizer can be mixed with the natural gas circulating in the pipe 102. The liquid fraction collected at the bottom of the de-ethanizer is sent to a de-propanizer.

The gas circulating in the pipe 102 and the second cooling stream circulating in the pipe 202 enter into the other exchanger E2 in order to circulate therein in parallel and co-current directions.

The second cooling stream exiting the exchanger E2 via the pipe 201 is relieved by the relief component T3. The relief component T3 can be a turbine, a valve or a combination of a turbine and a valve. The second relieved cooling stream originating from the turbine T3 is sent by the pipe 203 into the exchanger E2 in order to be vaporized while counter-current cooling the natural gas and the second cooling stream.

At the outlet of the exchanger E2, the second vaporized cooling stream is compressed by the compressor K2, then cooled in the indirect heat exchanger C2 by exchanging heat with an external cooling fluid, for example, water or air. The second cooling stream originating from the exchanger C2 is sent into the exchanger E1 via the pipe 20. The pressure of the second cooling stream when exiting the compressor K2 can range between 2 MPa and 8 MPa. The temperature of the second cooling stream at the outlet of the exchanger C2 can range between 10° C. and 45° C.

In the method described with reference to FIG. 4, the second cooling stream is not split into separate fractions, but, in order to optimize the approach into the exchanger E2, the second cooling stream also can be separated into two or three fractions, with each fraction being relieved at a different pressure level, then sent to different stages of the compressor K2.

The second cooling stream is formed, for example, by a mixture of hydrocarbons and nitrogen, such as a mixture of methane, ethane and nitrogen, but can also contain propane and/or butane. The mole fraction proportions (%) of the components of the second cooling mixture can be:

-   -   Nitrogen: 0% to 10%;     -   Methane: 30% to 70%;     -   Ethane: 30% to 70%;     -   Propane: 0% to 10%;

The natural gas exits the heat exchanger E2 via the pipe 101 in a liquefied state at a temperature that is preferably at least 10° C. higher than the bubble point temperature of the liquefied natural gas produced at atmospheric pressure (the bubble point temperature denotes the temperature at which the first vapor bubbles form in a liquid natural gas at a given pressure) and at a pressure that is identical to the inlet pressure of the natural gas, to the nearest pressure losses. For example, the natural gas exits the exchanger E2 at a temperature ranging between −105° C. and −145° C. and at a pressure ranging between 4 MPa and 7 MPa. Under these temperature and pressure conditions, the natural gas does not remain entirely liquid after expansion up to atmospheric pressure.

Of course, the invention is not limited to the particular examples described and illustrated in the present application. Other alternative forms or embodiments within the competence of a person skilled in the art may also be contemplated without departing from the scope of the invention. For example, other configurations for injecting and extracting fluids into and out of the exchanger, other directions of flow of the fluids, other types of fluids, etc., obviously can be contemplated, depending on the constraints stipulated by the method to be implemented.

It will be understood that many additional changes in the details, materials, steps and arrangement of parts, which have been herein described in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims, Thus, the present invention is not intended to be limited to the specific embodiments in the examples given above. 

1.-12. (canceled)
 13. A heat exchanger comprising a plurality of plates parallel to a longitudinal direction and together defining a first series of passages for the flow of at least one refrigerant configured to exchange heat with at least one calorigenic fluid, at least one passage of the first series defined between two adjacent plates comprising: a refrigerant inlet configured to introduce the refrigerant into a portion of said passage and a refrigerant outlet configured to discharge the refrigerant from the portion,  wherein the at least one passage of the first series further comprises: at least one other refrigerant inlet configured to introduce another refrigerant into another portion of said passage and at least one other refrigerant outlet is configured to discharge the other refrigerant from the other portion, said other inlets and outlets being arranged so that said at least one passage of the first series is divided, in the longitudinal direction, into at least said portion for the flow of the refrigerant and said other portion for the flow of the other refrigerant.
 14. The exchanger as claimed in claim 13, wherein a plurality of passages of the first series each comprises at least one inlet, one outlet, one other inlet and one other outlet for refrigerant, said inlets being fluidly connected to the same inlet manifold, said other inlets being fluidly connected to another identical inlet manifold, said outlets being fluidly connected to an identical outlet manifold and said other outlets being fluidly connected to another identical outlet manifold.
 15. The exchanger as claimed in claim 13, further comprising a first end, in the vicinity of which, during operation, the temperature is the lowest of the exchanger, and a second end, in the vicinity of which, during operation, the temperature is the highest of the exchanger, said second end being arranged downstream of the first end in the longitudinal direction, the portion for the flow of the refrigerant being arranged alongside the first end and the other portion for the flow of the other refrigerant being arranged between the portion and the second end.
 16. The exchanger as claimed in claim 13, wherein the plates together define a second series of passages for the flow of at least one calorigenic fluid, at least one passage of the second series being adjacent to said at least one passage of the first series and being configured so that, when a stream of calorigenic fluid circulates in said passage, said stream of calorigenic fluid exchanges heat with the refrigerant in the vicinity of at least part of the portion and with the other refrigerant in the vicinity of at least part of the other portion.
 17. The exchanger as claimed in claim 16, wherein at least one passage of the second series comprises, in the vicinity of the second end of the exchanger, a calorigenic fluid inlet configured to distribute the calorigenic fluid in said at least one passage and, in the vicinity of the first end of the exchanger, an outlet configured to discharge the calorigenic fluid from said at least one passage.
 18. The exchanger as claimed in claim 13, wherein said at least one passage of the first series comprises at least two other inlets is configured to respectively introduce at least two other refrigerants into at least two other respective portions of said passage and two other outlets configured to respectively discharge the at least two other refrigerants from the at least two other portions, said two other inlets and two other outlets being arranged so that said at least one passage of the first series is divided, in the longitudinal direction, into at least said portion and into said two other portions.
 19. A method for exchanging heat implementing a heat exchanger as claimed in claim 13, the method comprising: i) introducing a stream of calorigenic fluid into at least one passage of a second series of passages defined between the plates of the exchanger; ii) introducing a refrigerant via the inlet of at least one passage of the first series; iii) discharging the refrigerant introduced in step ii) via the outlet of said passage; iv) introducing at least one other refrigerant via the other inlet of said passage; v) discharging the other refrigerant introduced in step iv) via the other outlet of said passage, said stream of calorigenic fluid exchanging heat at least with the refrigerant and with the other refrigerant.
 20. The method as claimed in claim 19, wherein the refrigerant discharged in step iii) exits the exchanger at a first temperature and the other refrigerant introduced in step iv) enters the exchanger at a second temperature, with the second temperature being lower than the first temperature.
 21. The method as claimed in claim 20, wherein the second temperature is at least 1° C. lower than the first temperature. 